This invention relates to a filter cartridge construction suitable for filtering liquids and gases.
At the present time a wide variety of filter cartridge constructions are utilized to purify fluids. These cartridge constructions are designed to remove solids and colloidal particles as well as microorganisms. The basic two separate and distinct types of cartridges used in filtration of gases and liquids are depth filters (typically wound) and surface or screen filters (usually pleated). A depth filter is primarily used to remove most of the contaminants and particles. It is typically utilized upstream of a surface or screen filter. The most important properties for a depth filter are its "dirt holding capacity" or throughput, pressure drop and retention. The filter design allows contaminants and particles to be trapped in stages within the depth of the filter due to the construction of the multiple layers of various media types. A wound depth filter has multiple layers with the most open media (largest micron retention rating) i.e. largest pore size usually the outermost layer, adjacent the liquid inlet with the tightest media (smallest micron retention rating, i.e. smallest pore size) adjacent the liquid outlet. The tightest media at the core adjacent the liquid outlet will have the least amount of surface area due to the smallest diameter around which it is wrapped. This layer at the core contributes to most of the pressure drop of the cartridge because the media has the highest pressure drop and the least amount of filtration surface area. Likewise, this layer will significantly reduce the capacity of the filter due to both the low filtration surface area and the smallest micron retention rating.
A surface or screen filter will retain virtually 100% of the particles or contaminants for which it is rated. The media used in a surface or screen filter typically has a high pressure drop and low "dirt holding capacity" or throughput because of its high retention efficiency. The media normally used in a surface filter comprises glass or polymeric microfibers. The normally used medium in a screen filter comprises a polymeric microporous membrane. Particles are retained by size exclusion primarily on the surface of the screen filter rather than within the depth of the filter. Particles are retained primarily on the surface of a surface filter which has a controlled pore size. Particles smaller than the controlled pore size tend to be trapped within the media of the surface filter. However as a result of the controlled pore structure, they provide more predictable filtration than depth filters. For the surface or screen filter to be economical, the media is pleated to obtain a large filtration surface area. Presently, the wound depth filter and the surface or screen filter are utilized in series in separate housings to effect the desired level of purification.
The useful life of a filter construction is the time the filter construction functions to remove particles of a size for which it is rated while avoiding a desired maximum pressure drop between the inlet and outlet of the filter construction. When the filter construction either fails to retain particles of a size for which it is rated or a pressure drop larger than desired results due to particle retention within the filter construction, it is replaced.
It is economically desirable to maximize the useful life of a filter construction. Convenient measures of useful life of a filter construction are the volume of fluid satisfactorily filtered prior to experiencing a pressure drop between its inlet and outlet which equals or exceeds the maximum pressure drop desired.
It is also desirable to minimize the volume of filter used to attain satisfactory filtration for a given time in order to minimize the cost of filter material. In addition, it is desirable to utilize a single filter housing rather than a plurality of filter housings in order to minimize filter housing cost and to eliminate the need for conduits and associated structures such as valves for connecting the housings. While a single housing is desirable, the filter construction positioned therein must be capable of retaining particles equal to and larger than a desired size while affording an economically satisfactory useful life.
It has been proposed in U.S. Pat. No. 3,552,553 to provide a dual media filtration cartridge formed from a pleated filtration medium positioned adjacent a liquid outlet from the cartridge and a porous foam medium adjacent a liquid inlet to the cartridge. The foam medium surrounds the pleated medium. This filter cartridge is undesirable since the foam medium has a random porosity which results in a random micron retention characteristic throughout the foam layer. This micron retention characteristic is undesirable since retained particles tend to be located within a narrow stratum within the foam which results in a portion of the foam thickness not being utilized for filtration.
It has also been proposed in U.S. Pat. No. 3,334,752 to provide a dual filter construction in a single housing. The filter construction comprises a pleated paper tube filter positioned on a hollow core and an upstream filter comprising napped wound yarn which surrounds the pleated filter. The wound yarn portion functions to remove slimy constituents while the pleated paper filter functions to remove even minute solid particles. The yarn, which has the same porosity through the depth of the upstream filter, is wound to form rhombic openings which diminish in cross-sectional area when proceeding from an outermost surface to the surface of the upstream filter positioned adjacent the pleated filter. Thus, the yarn utilized does not have progressive lower micron retention characteristics as one progresses toward the pleated filter.
It would be desirable to provide a filter cartridge construction which avoids the undesirably high pressure drop such as that due to the tightest filtration medium positioned adjacent the liquid outlet from the filter cartridge. Furthermore, it would be desirable to provide a filter cartridge having a significantly improved useful life as compared with filter cartridge of the prior art having at least about the same retention efficiency. Such filter cartridges would provide substantially improved economic benefit from the standpoint of increased satisfactory throughput fluid volume capacity while minimizing the cost of filter cartridge support apparatus to achieve this capacity.